1. Field of the Invention
This invention relates to displays such as liquid crystal displays, and particularly to electrophoretic displays that display images using display media such as electrophoretic ink in accordance with electrophoretic effects.
2. Description of the Related Art
Conventionally, electrophoretic effects are well known among scientists and engineers, wherein charged particles dispersed in a fluid or liquid medium move under the influence of an electric field. As an example of the application of the electrophoretic effects, engineers try to realize displays by using charged pigment particles that are dispersed and contained in dyed solution arranged between a pair of electrodes. Under the influence of an electric field, the charged pigment particles are attracted to one of the electrodes, so that desired images will be displayed. The dyed solution in which charged pigment particles are dispersed is called electrophoretic ink, and the display using the electrophoretic ink is called an electrophoretic display (abbreviated as ‘EPD’).
Each of the charged pigment particles has a nucleus that corresponds to a rutile structure such as TiO2, for example. The nucleus is covered by a coating layer made of polyethylene, for example. As solvents, it is possible to use a solution dissolving ethylene tetrachloride, isoparaffin, and anthraquinone dye, for example. The charged pigment particles and the solvents each have different colors. For example, the charged pigment particles are white, while the solvents are blue, red, green, or black, for example. At least one of the electrodes is formed as a transparent electrode.
Applying an electric field to the electrophoretic ink externally, if the pigment particles are negatively charged, they move in a direction opposite to a direction of the electric field. Thus, the display produces a visual representation such that one surface of the display being observed through the electrophoretic ink seems to be colored in either the color of the solvent or the color of the charged pigment particles. By controlling the movement of charged pigment particles in each pixel, it is possible to represent visual information on the display surface of the display.
The solvent and the charged pigment particles both have approximately the same specific gravity. For this reason, even if the electric field disappears, the charged pigment particles can maintain their positions, which are fixed by the application of the electric field, for a relatively long time, which may range from several minutes to twenty minutes, for example. Because of the aforementioned property of the charged pigment particles of the electrophoretic ink, it is possible to anticipate low power consumption by the electrophoretic display. In addition, the electrophoretic display is advantageous because of the high contrast and very large viewing angle, which reaches approximately ±90 degrees. Generally speaking, a human observer is inevitably required to directly view colors of pigments and/or colors of dyes in the electrophoretic display. Whereas the liquid crystal display of the transmission type requires the human observer to view light from fluorescent tubes of the back light, the electrophoretic display can produce visually subtle colors and shades, which are gentle on the human eyes. In addition, the electrophoretic ink is inexpensive compared to liquid crystal. Further, the electrophoretic display does not need a back light. Therefore, it is anticipated that electrophoretic displays can be manufactured at the relatively low cost.
In spite of the aforementioned advantages, manufacturers could not actually produce electrophoretic displays for practical use because of low reliability in operation due to cohesion of charged pigment particles. However, recent advances in technology have shown that the reliability can be improved by using microcapsules filled with electrophoretic ink. Therefore, electrophoretic displays have recently suddenly become a focus of interest.
Various papers and monographs have been written with regard to concrete examples of displays using electrophoretic ink encapsulated in microcapsules. For example, it is possible to list two papers, namely, a first paper entitled “44.3L: A Printed and Rollable Bistable Electronic Display” that is written by P. Drzaic et al for the SID 98 DIGEST 1131, and a second paper entitled “53.3: Microencapsulated Electrophoretic Rewritable Sheet” that is written by H. Kawai et al for the SID 99 DIGEST 1102.
The aforementioned first paper describes that four types of layers are sequentially printed on a polyester film, that is, a transparent conductive plate, an encapsulated electrophoretic ink layer, a patterned conductive layer of silver or graphite, and an insulation film layer. In short, the first paper proposes a ‘flexible’ display in which a hole (or holes) is open on the insulating film to allow designation of an address (or addresses) for the patterned conductive layer and to allow provision of a lead line (or lead lines). The second paper proposes a rewritable sheet that operates based on the electrophoresis by using the microencapsulated electrophoretic ink, and it also proposes a method for writing information onto the sheet. In addition, it is possible to propose a display in which a surface of an active-matrix type array of elements such as the low-temperature processed polysilicon thin-film transistors (TFT) is coated with the electrophoretic ink. Thus, it is possible to provide the ‘visually subtle and gentle’ display that is also reduced in consumption of electricity.
FIG. 1 shows a structure of the selected section of the electrophoretic display with respect to each pixel. The display uses two substrates 111 and 112, which are fixed by bonding and are arranged opposite to each other. A common electrode 113 is formed just below the substrate 112, under which a pixel electrode 114 is formed. An electrophoretic ink layer 115 containing plenty of microcapsules of electrophoretic ink is formed between the common electrode 113 and the pixel electrode 114. The pixel electrode 114 is connected to a drain electrode 117 of a thin-film transistor (TFT) 116 in series. The TFT 116 plays a role as a switch. At least one of the common electrode 113 and pixel electrode 114 is made by a transparent electrode, which corresponds to a display surface to be visually observed by a person or human operator.
The TFT 116 contains a source layer 119, a channel 120, a drain layer 121, and a gate insulation film 122 that are formed on an embedded insulation film 118. In addition, it also contains a gate electrode 123 formed on the gate insulation film 122, a source electrode 124 formed on the source layer 119, and a drain electrode 117 formed on the drain layer 121. Further, the TFT 116 is covered with an insulation film 125 and another insulation film 126 respectively.
Next, the internal structure and operation of the electrophoretic ink layer 115 will be described with reference to FIGS. 2A to 2C. The electrophoretic ink layer 115 is formed by a transparent binder 211 having light transmittance and plenty of microcapsules 212. The microcapsules 212 are distributed uniformly in the inside of the binder 211 in a fixed state. The thickness of the electrophoretic ink layer 115 is 1.5 to 2 times as large as external diameters of the microcapsules 212. As the material for the binder 211, it is possible to use silicone resin and the like. Each microcapsule 212 is defined by a capsule body 213 that has a hollow spherical shape and transmits light. The inside of the capsule body 213 is filled with liquid (or solvent) 214, in which negatively charged particles 215 are dispersed. Each of the charged particles 215 has a nucleus 216 that is coated with a coating layer 217. Each charged particle 215 and the liquid 214 mutually differ from each other in color. That is, different colors are set to them respectively. For example, the charged particles 215 are white, while the liquid 214 is blue, red, green or black. Additionally, approximately the same specific gravity is set for both of the liquid 214 and charged particles 215 within the microcapsule 212.
When an electric field is applied to the microcapsules 212 externally, the charged particles 215 move within the microcapsules 212 in directions opposite to the direction of the electric field. If the display surface of the display presently corresponds to an upper surface of the substrate 112 shown in FIG. 1, the charged particles 215 move upwards within the microcapsules 212 of the electrophoretic ink layer 115, which is shown in FIG. 2B. In that case, it is possible to observe the color (i.e., white) of the charged particles 215 that are floating upwards above the background color, which corresponds to the color (e.g., blue, red, green, or black) of the liquid 214. In contrast, if the charged particles 215 move downwards due to the application of an electric field to the microcapsules 212 of the electrophoretic ink layer 115 shown in FIG. 1, the display allows only the color (e.g., blue, red, green, or black) of the liquid 214 to be observed, which is shown in FIG. 2C. Once the charged particles 215 are moved in directions opposite to the direction of the electric field applied to the microcapsules 212, they will likely maintain the same positions within the microcapsules 212 for a relatively long time after the electric field disappears because they have approximately the same specific gravity as that of the liquid 214. That is, once the color of the charged particles 215 or the color of the liquid 214 appears on the display surface, it is maintained for several minutes or several tens of minutes. In short, the electrophoretic display has a memory for retaining colors of images. Therefore, by controlling the application of an electric field with respect to each of the pixels, it is possible to provide specific electric-field application patterns, by which information is to be displayed. Once the information is displayed on the display surface of the electrophoretic display, it is maintained on the display surface for a relatively long time.
However, the following problems are left unsolved in the electrophoretic display that is made by the combination of the electrophoretic ink and active-matrix type array of elements.
The drive voltage (or potential difference) that is needed for changing over the display content depends upon the sizes (i.e., diameters) of the microcapsules, and it is estimated to be 1 V/ m or so. Generally, the microcapsules have prescribed diameters that range within several tens of microns, for example. In consideration of the prescribed diameters of the microcapsules, the drive voltage is estimated at 10V or so. Suppose that the drive voltage is set to 10V in the electrophoretic display, which is driven by the known drive method typically employed by liquid crystal displays. In addition, the constant voltage of 10V is applied to the common electrode 113, while the prescribed voltage of 0V or 20V is applied to the pixel electrode 114. That is, the prescribed voltage applied to the pixel electrode 114 (hereinafter, simply referred to as ‘pixel electrode drive voltage’) is set to 0V in order to increase the potential of the common electrode 113 to be higher than the potential of the pixel electrode 114. The pixel electrode drive voltage is set to 20V in order to increase the potential of the pixel electrode 114 to be higher than the potential of the common electrode 113. Switching over the pixel electrode drive voltage allows the electrophoretic display to rewrite its display content. Actually, the TFT 116 is used to switch over the pixel electrode drive voltage. In practice, however, if the electrophoretic display is driven as described above, the drive voltage is increased too high to ensure satisfactory reliability in the switching operation of the TFT 116. In addition, the pixel electrode drive voltage of 20V is only the least estimated voltage. In other words, an electrophoretic display for practical use may have an increased pixel electrode drive voltage of 30V or more. If the pixel electrode drive voltage is increased very high, it becomes more difficult to ensure satisfactory reliability in the switching operation of the TFT.
Another typical drive method for use in liquid crystal displays is to vary the potential of the common electrode as well, which is normally called ‘common voltage swing’. Specifically, the pixel electrode drive voltage is set to 0V while the voltage applied to the common electrode (hereinafter, simply referred to as ‘common electrode drive voltage’) is set to 10V in order to increase the potential of the common electrode to be higher than the potential of the pixel electrode. Alternatively, the pixel electrode drive voltage is set to 10V while the common electrode drive voltage is set to 0V in order to increase the potential of the pixel electrode to be higher than the potential of the common electrode. Adequately switching over the pixel electrode drive voltage and common electrode drive voltage allows the electrophoretic display to rewrite its display content. Thus, it is possible to improve the reliability in the switching operation of the TFT.
The aforementioned drive method has a problem, which will be described below.
Suppose that the common electrode drive voltage is set to 10V while the pixel electrode drive voltage is set to 0V in order to rewrite the display content with respect to a certain pixel of the display. In order to prevent other pixels from being mistakenly rewritten in display content, the voltage of 10V should be applied to all other pixel electrodes of the display. Normally, the voltage is applied to the pixel electrodes by sequentially selecting transistors for the pixels. Therefore, it is difficult to perfectly match the voltage applying timing for the prescribed pixel electrode with the voltage applying timing for the common electrode. For this reason, a delay may be caused to occur between these timings. Due to such a delay, there is a possibility that an error will occur in rewriting the display content of the display. Although the appropriate voltage is applied to the pixel electrode at a good timing before the occurrence of an error in rewriting the display content, there is still a possibility that an error will occur in rewriting the display content because of the gradual reduction of the voltage applied to the pixel electrode, which is caused by electromagnetic leakage from the pixel transistor.